Flood Zone 3a
FZ3aLand assessed as having a 1% or greater annual probability of river flooding, or a 0.5% or greater annual probability of sea flooding -- the high-probability flood zone.
Flood Zone 3a is the high-probability flood zone, covering land where the annual probability of river flooding is 1% or greater (1 in 100 or more frequent), or tidal flooding is 0.5% or greater (1 in 200 or more frequent). Development in this zone faces the most stringent planning requirements short of the functional floodplain (Flood Zone 3b).
A comprehensive Flood Risk Assessment is mandatory, and both the Sequential Test and Exception Test must typically be satisfied. The FRA must demonstrate that the development will be safe for its lifetime, including the effects of climate change, and that it will not increase flood risk elsewhere.
The NPPF restricts certain development types in Flood Zone 3a:
- “Highly vulnerable” uses (such as basement dwellings, caravans, and emergency services) are not permitted
- “More vulnerable” uses (residential) require the Exception Test
- “Less vulnerable” uses (commercial) and water-compatible development are generally appropriate
Site-specific hydraulic modelling is almost always required in Flood Zone 3a to determine actual flood depths, velocities, and hazard ratings. This modelling informs the design of mitigation measures including raised finished floor levels, flood-resilient construction, and safe access and egress routes.
Aegaea’s in-house flood modelling team works with industry-standard software such as TUFLOW, HEC-RAS, and Flood Modeller to provide the detailed analysis required.